Cochlear stimulation device

ABSTRACT

A cochlear stimulation device comprising an electrode array designed to provide enhanced charge injection capacity necessary for neural stimulation. The electrode array comprises electrodes with high surface area or a fractal geometry and correspondingly high electrode capacitance and low electrical impedance. The resultant electrodes have a robust surface and sufficient mechanical strength to withstand physical stress vital for long term stability. The device further comprises wire traces having a multilayer structure which provides a reduced width for the conducting part of the electrode array. The cochlear prosthesis is attached by a grommet to the cochleostomy that is made from a single piece of biocompatible polymer. The device, designed to achieve optimum neural stimulation by appropriate electrode design, is a significant improvement over commercially available hand-built devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.,60/986,549 “Cochlear Stimulation Device”, filed Nov. 8, 2007, thedisclosure of which is incorporated herein by reference. Thisapplication is related to and incorporates by reference the followingcommonly assigned patent applications: 2004/0220652, filed Nov. 4, 2004for Adherent Metal Oxide Coating Forming a High Surface Area Electrode;2006/0247754, filed Nov. 2, 2006 for Flexible Circuit Electrode Array;2007/0092750, filed Apr. 26, 2007 for Electrode Surface Coating andMethod for Manufacturing the Same; and 2008/0221653, filed Sep. 11,2008, for Flexible Circuit Electrode Array.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to cochlear stimulation device, thatimproves a cochlear electrode array of the implanted portion of thesystem.

BACKGROUND OF THE INVENTION

Human deafness results from numerous factors including trauma, earinfections, congenital factors, toxic effects of some antibiotics, andfrom diseases such as meningitis. Sensorineural damage (damage to thehair cells in the cochlea) is the largest single form of hearing loss.In a healthy ear these hair cells convert acoustic signals in the innerear to electrical signals that can be interpreted by the brain as sound.It is estimated that over 7% of the U.S. population is affected bysensorineural deafness, and one in a thousand infants is born totallydeaf. Extrapolating these percentage figures, it is estimated that thereare 30 million people in the world who are profoundly deaf.

Considerable research over the past several decades has been directedtowards developing a means to bypass the non-functioning hair cells inthe inner ear (or cochlea) by using electrodes to directly stimulateauditory afferent neurons within the cochlea. This so called cochlearimplant technology has progressed from early methods of attaching one ormore single wire electrodes onto the promontory or the bony shell of thecochlea, to drilling directly into the cochlea, and inserting electrodesinto the scalae therein. Electrodes used in modern cochlear prosthesesgenerally use a longitudinal monopolar (or bipolar) electrodeconfiguration where small platinum/iridium plates or circular platinumrings connected internally by thin wires, with the electrodes and wiresheld together in a smooth elongated silicone carrier, are surgicallyimplanted into the scala tympani (one of the canals within the cochlea),via a hole made in the mastoid bone behind the ear. Entry into the scalatympani is generally via the round window membrane. The electrodes areelectrically connected to an electronics package anchored in a cavitymade in the mastoid bone. Information is sent to this internalelectronics package transcutaneously, via RF transmission across theskin barrier, from an external body-worn (generally behind the ear)electronics package that houses the speech processor, controlelectronics and power supply.

Current cochlear implant systems include an implant portion and anexternal portion. The implant portion typically includes: (1) anelectrode array, (2) an implanted coil and (3) a hermetically-sealedhousing to which the electrode array and implanted coil are attached andin which electronic circuitry, e.g., data processing circuitry and pulsegenerator circuitry are housed. The external portion typically includes:(1) a microphone, (2) a battery-powered sound processor for processingthe signals sensed by the microphone and for generating control andother signals that are transmitted to the implant portion and (3) aheadpiece, connected to the sound processor by way of a cable orwire(s), in which an external coil is housed. In operation, theheadpiece coil (external coil) is inductively coupled with the implantedcoil so that power and data can be transferred to the implant portionfrom the external portion.

U.S. Pat. No. 5,123,422 teaches the use of internal hinges or slits,where such hinges or slits are oriented to give flexibility in only oneplane, and can be inserted in the scala tympani without curling, thusorienting the electrode sites “to obtain good stimulation of the nervecells”. U.S. Pat. No. 4,261,372 uses “V” shaped notches along one sideof the array to permit the array to assume the required curved shapewithin the scala, and to obtain greater insertion depth of theelectrodes by first inserting one part of the electrode into the firstturn of the scala tympani and then inserting the other part into thesecond turn of the scala tympani. U.S. Pat. No. 4,832,051 describes anelectrode device where “the elements are resiliently attached togetherso that the stack of elements is stiff in compression along the commonaxis and is flexible in tension.” Cochlear stimulation devises have beenfurther described in U.S. Pat. Nos. 7,194,314, 7,085,605, 6,906,262,6,782,619, 6,678,564, or 6,374,143.

The electrode is an important part of cochlear implant system because itaffects the current spread and the response of the auditory nerves.Modern technology uses multi-channel (electrode) implants as opposed tosingle electrode implants, as the former provides electrical stimulationat multiple sites in the cochlea using an array of electrodes. Anelectrode array is used so that different auditory nerve fibers can bestimulated at different places in the cochlea, thereby exploiting theplace mechanism for coding frequencies. Different electrodes arestimulated depending on the frequency of the signal. Electrodes near thebase of the cochlea are stimulated with high frequency signals, whileelectrodes near the apex are stimulated with low frequency signals. Thedesign of the electrode array are important with regard to the electrodeplacement, number of electrodes and spacing between electrodes,orientation of electrodes with respect to the excitable tissue andelectrode configuration. The electrodes are commonly placed in the scalatympani because it brings the electrodes in close proximity withauditory neurons which lie along the length of the cochlea and therebypreserves the place mechanism for coding frequencies. The larger thenumber of electrodes, the finer the place resolution for codingfrequencies. However, using a large number of electrodes will notnecessarily result in better performance, because frequency coding isconstrained by the number of surviving auditory neurons that can bestimulated. Studies have shown that for adequate speech perception, atleast 8 electrodes are required.

Commercially available cochlear implant devices comprise simple taperedlongitudinal bipolar and monopolar electrode arrays using smallplatinum/iridium balls or circular rings. However these devices weredeveloped taking into consideration the ease of fabrication as well assurgical insertibility rather than the critical design parametersnecessary to achieve optimum neural stimulation. They continue to behand built under a microscope and are made using wire basedtechnologies. There is, therefore, still a need in the art to improvethe currently available cochlear stimulation devices by appropriateelectrode design consideration.

The present invention is an improvement over the known commercialdevices. The device comprises an electrode array designed to increasethe current transfer capability of the electrodes by using high surfacearea electrodes without increasing the geometrical surface area. Sincean implantable stimulation or sensing electrode is intended for longterm use in a neural stimulator with a low power consumption and limitedcompliance voltages, it requires high electrode capacitance andcorrespondingly low electrical impedance. Without sufficiently lowimpedance, a large voltage may cause polarization of both the electrodeand the tissue to which the electrode is attached forming possibleharmful byproducts, degrading the electrode and damaging the tissue.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a cochlear stimulation device invarious embodiments. The device comprises an electrode array suitablefor attaching to the cochlea, wire traces and a polymer body.

A layer of polymer is laid down, commonly by some form of chemical vapordeposition, spinning, meniscus coating or casting. A layer of metal,preferably platinum, and more preferably platinum grey, which has afractal geometry, is applied to the polymer and patterned to createelectrodes and leads for those electrodes. The electrodes can containPt, Ir, Au, Ru, Rh, Pd, C, conducting polymers or alloys or oxidesthereof. In an alternative embodiment, the platinum grey fractal surfaceis coated with either a gradient or discrete coating of an inertmaterial, such as iridium oxide. The electrodes have a rough surface andhence a very large surface area when compared to an electrode with asmooth metal surface having the same geometric shape. Because of therough surface area, the electrodes have sufficient physical andmechanical strength to withstand physical stress. Additionally, theiridium oxide layer provides very high charge storage capacity for pulsestimulation. The method of making a high surface area coating isdescribed in US Patent Application 2004/0220652 and 2007/0092750 both ofwhich are assigned to the same assignee as is the present application,and which are incorporated herein by reference.

The novel feature of the invention is a cochlear stimulation devicecomprising an electrode array with a high surface area electrodes andmetal leads which have a multilayer structure, preferably two or threelayers. This variation allows a reduced width of the conducting cable ofthe electrode array. The advantage is that the electrode density isincreased without increasing the array cable width or even with reducedcable width. The conducting part can be made narrower by slight increaseof the thickness.

The method of making an flexible electrode array as applied to retinalstimulation is described in US patent Application US 2006/0247754 whichis assigned to the same assignee as is the present application, andwhich is incorporated herein by reference.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a cochlear stimulationdevice comprising an electrode array wherein the surface area of theindividual electrodes in the array is high for a given geometricalelectrode size thereby enhancing the charge transfer capability of theelectrodes.

It is also an object of the present invention to use a series of newelectrode geometries that control charge transfer characteristics by thestrategic use of edges and corners to concentrate current delivery.

It is also an object of the present invention to use a multilayerstructure for the metal traces. The traces require this way a smallerwidth than traces without multi leads. In one embodiment, the two leadsor traces on the right and left side of the electrode array lead toelectrodes preferably on different levels of the array. In anotherembodiment, the upper half and the lower half are cut vertically and arein two different layers. In yet another embodiment, the two halves areplaced next to each other. The advantage of the present multilayer isthat the conductivity stays stable because even if the electrode startsto dissolve at the edge to the insulating material the conductivitystays stable until the electrode dissolves completely towards thecenter. Vias can be used to connect traces on different metal layers orto connect an electrode to a metal a trace.

Other objects, advantages and novel features of the present inventionwill become apparent from the following description of the inventionwhen considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The preferred and alternative embodiments of the invention will bedescribed by references to the accompanying drawings, in which:

FIG. 1 shows a top view of a cochlear electrode array with circularshaped electrodes according to the present invention.

FIG. 1A shows a cross sectional view of a cochlear electrode arrayaccording to the present invention.

FIG. 2 shows a top view of a cochlear electrode array with square shapedelectrodes according to the present invention.

FIG. 3 shows a top view of a cochlear electrode array according to thepresent invention with a return/ground electrode on the entry level.

FIG. 4 shows a cross sectional view of a cochlear electrode arrayaccording to the present invention with a return/ground electrode on theentry level.

FIG. 5 shows a cross sectional view of a human ear.

FIG. 6 shows a cross sectional view of a cochlear electrode arrayaccording to the present invention as implanted in the cochlea.

FIG. 7 shows a cross sectional view of a cochlear electrode arrayaccording to the present invention as implanted in the cochlea.

FIG. 8 shows a cross sectional view of a cochlear electrode arrayaccording to the present invention with elevated electrodes.

FIG. 8A shows a cross sectional view of a cochlear electrode array withrecessed electrodes and silicone bumps between the electrodes accordingto the present invention with elevated electrodes.

FIG. 8B shows a top view of a cochlear electrode with multiple leadsaccording to the present invention with elevated electrodes.

FIG. 8C shows a top view of a cochlear electrode with another variationof the multi lead trace wires, wherein the two halves are placed next toeach other (176A layer 1 and 176B layer 2).

FIG. 9 shows of top view of a cochlear electrode array with acochleostomy grommet according to the present invention.

FIG. 10 shows a cross sectional view of a cochlear electrode array witha cochleostomy grommet according to the present invention.

FIG. 11 shows a cross sectional view of a cochleostomy grommet accordingto the present invention.

FIG. 12 shows of top view of a cochlear electrode array with acochleostomy grommet in place according to the present invention.

FIG. 13 shows a top view of a cochlear electrode array with a softsilicone tip according to the present invention.

FIG. 14 shows a cross sectional view of a cochlear electrode array witha soft silicone tip according to the present invention.

FIG. 15 shows a cross sectional view of a multilayer cochlear electrodearray according to the present invention.

FIG. 16 shows a cross sectional view of a multilayer cochlear electrodearray according to the present invention.

FIG. 17 shows a cross sectional view of a multilayer cochlear electrodearray with interlayer vias to allow connections between the metal tracesaccording to the present invention.

FIG. 18 shows top view of a cochlear prosthesis silicone molded into ahelical shape.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE OF PRACTISING THEINVENTION

Polymer materials are useful as electrode array bodies for neuralstimulation. They are particularly useful for retinal stimulation tocreate artificial vision, cochlear stimulation to create artificialhearing, or cortical stimulation for many purposes. Regardless of whichpolymer is used, the basic construction method is the same. A layer ofpolymer is laid down, commonly by chemical vapor deposition, spinning,meniscus coating or casting. A layer of metal, preferably platinum, isapplied to the polymer and patterned to create electrodes and leads forthose electrodes. Patterning is done by photolithographic methods. Asecond layer of polymer is applied over the metal layer and patterned toleave openings 200 for the electrodes, or openings 200 are created laterby means such as laser ablation. Hence the array and its supply cableare formed of a single body. Alternatively, multiple alternating layersof metal and polymer are applied to obtain more metal traces within agiven width.

To achieve the accurate place coding of frequency throughmultiple-electrode stimulation, the electrodes have to be placed belowthe sense organ for hearing (scala tympani) and close to the ganglioncells at the center of the inner ear. But placement of the electrodes inthe inner ear could possibly damage the very nerves that were intendedto stimulate. In order to minimize the risk of surgical trauma to thenerves, the electrodes should have the right mechanical properties. Theyneed to be smooth, tapered and flexible at the tip and stiffer towardsthe proximal end. The edges of a flexible circuit polymer array may bequite sharp and cut the delicate neural tissue. Common flexible circuitfabrication techniques such as photolithography, generally require thata flexible circuit electrode array be made flat. With most polymers, itis possible to curve them when heated in a mold. By applying the rightamount of heat to a completed array, a helical shape can be induced toapproximate the shape of the cochlea. With a thermoplastic polymer suchas liquid crystal polymer, it may be further advantageous to repeatedlyheat the flexible circuit in multiple molds, each with a decreasingradius. Further, it is advantageous to add material along the edges of aflexible circuit array. Particularly, it is advantageous to add materialthat is more compliant than the polymer used for the flexible circuitarray.

FIGS. 1, 1A, 2, 6 and 7 show application of the present invention to acochlear prosthesis. FIG. 1 shows of top view of cochlear electrodearray 110. The cochlear electrode array 110 tapers toward the top to fitin an ever smaller cochlea and because less width is required toward thetop for patterned high density metal traces 172. The electrodes 174 arearranged linearly along the length of the array 110. Further a skirt 160of more compliant polymer, such as silicone surrounds the array 110.FIG. 1A is a cross sectional view of the cochlear electrode array 110.The cochlear electrode array 110 includes a bottom polymer layer 170,patterned high density metal traces 172 and a top polymer layer 176.Openings in the top polymer layer 176 define electrodes 174.

The cochlear electrode array 110 is made flat as shown in FIGS. 1A, 6and 7. It is then thermoformed, as described above, into a spiral shapeto approximate the shape of the cochlea, as shown in FIGS. 5, 6 and 18.FIG. 18 shows how cochlear prosthesis can be molded in a silicone moldto maintain a helix structure. The cochlear electrode array 110 isimplanted with the bottom layer 170 formed towards the outside of thecurvature, and the top polymer layer 176 toward the inside of thecurvature. This is opposite of the thermoforming process used for aretinal array. A cortical array would be thermoformed to curve inwardlike a cochlear array.

FIG. 1 shows circular shaped electrodes as a preferable embodiment. FIG.2 shows square shaped electrodes as preferred embodiment. Circle shapes,star shapes, square shapes and rings can be used in the electrode array.Each of the shapes can contain overlapping edges and mesh grids. Thispresents a series of new electrode geometries that control chargetransfer characteristics by the strategic use of edges and corners toconcentrate current delivery. These designs are an improvement onconventional surface electrode designs which are typically circles. Theelectrodes can contain Pt, Ir, Au, Ru, Rh, Pd, C, conducting polymers oralloys or oxides thereof. The electrodes contain preferably platinum andmore preferably platinum gray.

Platinum gray's color density values range from 0.4 D to 1.3 D, whileplatinum black and shiny platinum both have color density values greaterthan 1.3 D.

Platinum gray can be distinguished from platinum black based on theadhesive and strength properties of the thin film coating of thematerials. Adhesion properties of thin film coatings of platinum grayand platinum black on 500 microns in diameter electrodes have beenmeasured on a Micro-Scratch Tester (CSEM Instruments, Switzerland). Acontrolled micro-scratch is generated by drawing a spherical diamond tipof radius 10 microns across the coating surface under a progressive loadfrom 1 millinewton to 100 millinewtons with a 400 micron scratch length.At a critical load the coating will start to fail. Using this test it isfound that platinum gray can have a critical load of over 60millinewtons while platinum black has a critical load of less than 35millinewtons.

Electrodes with a surface layer of platinum gray are prepared usingconstant voltage plating. The most preferable voltage range to produceplatinum gray has been found to be −0.45 Volts to −0.85 Volts. Applyingvoltage in this range to the plating solution (preferably 3 to 30 mMammonium hexachloroplatinate in disodium hydrogen phosphate) yields aplating rate in the range of about 1 micron per minute to 0.05 micronsper minute, the preferred range for the plating rate of platinum gray.Constant voltage control also allows an array of electrodes in parallelto be plated simultaneously achieving a fairly uniform surface layerthickness for each electrode. Surface area increase of platinum greycalculated from the electrode capacitance is about 50 to 500 times thesurface area resulting from the basic geometrical. This increasedelectrode surface area of platinum grey in the electrode array of thecochlear implant device significantly enhances charge injection capacitynecessary for stimulation. Further the robust surface of the resultantelectrode brought about by electroplating of platinum gray imparts themechanical strength that is vital for long term use of the cochleardevice.

Furthermore, it has been found that because of the physical strength andlow stress of platinum gray, it is possible to plate surface layers ofthickness greater than 30 microns. It is very difficult to plate shinyplatinum in layers greater than approximately several microns becausethe internal stress of the dense platinum layer which will cause theplated layer to peel off and the underlying layers cannot support theabove material. The additional thickness of the plate's surface layerallows the electrode to have a much longer usable life.

In an alternative embodiment, the platinum grey fractal surface iscoated with either a gradient or discrete coating of an inert material,such as iridium oxide. Iridium oxide coats the fractal surface of theplatinum gray with a cauliflower-like morphology with feature sizesranging from 0.5 to 15 microns. Each branch of such structure is furthercovered by smaller and smaller features of similar shape. The featuresparticles on the surface layer may be in the nanometer range. This roughand porous fractal structure increases the electrochemically activesurface area of the platinum surface when compared to an electrode witha smooth platinum surface having the same geometric shape. This iridiumoxide layer provides very high charge storage capacity for pulsestimulation. The most preferable voltage range to produce adherentiridium oxide has been found to be +0.45V to +0.65V. Applying voltage inthis range with the above solution 13 yields a plating rate of about 2to 4 mC/cm.sup.2/min, which is the preferred range for the plating rateof iridium oxide.

A comparison of the impedance spectra for different surfaces indicatesthat the electrode impedance decreased after rough platinum plating andwas further reduced after iridium oxide plating on the rough platinumsurface. The charge storage capacity measured in the electrode'scapacitance, which is proportional to the electrode surface area, wasdetermined to increase more than 200 times for the iridium oxide platedsurface, as compared with unplated electrodes of the same diameter.

The traces contain preferably Pt as conductive layer. Adhesion layerscontaining preferably titanium (Ti) can be applied on the top, thebottom or on both sides of the conductive layer. Such a sandwich layercontaining titanium/platinum/titanium (Ti/Pt/Ti) has a thickness ofabout 5000Å. The polyimide separation layer has a thickness of about 5μm.

FIG. 3 shows a top view of a cochlear electrode array according to thepresent invention with a return/ground electrode 150 on the entry level.FIG. 4 shows a cross sectional view of a cochlear electrode arrayaccording to the present invention with a return/ground electrode on theentry level. An electrode array must have a return, or common, electrodeto make a complete circuit with the neural tissue. It is advantageousthat the return electrode is large and in tissue less sensitive toelectrical stimulation to avoid stimulating tissue with the returnelectrode.

The return electrode is coupled by a cable to a contact pad forattaching the return electrode to the electronics package. FIG. 3 showsthe preferred electrode array with the return electrode 150 on front ofthe cable inside the eye. FIG. 4 is the preferred electrode array withthe return electrode 150 on the back of the cable outside the eye. Thereturn electrode 150 should be provided in a mesh, star pattern or hashpattern to reduce the eddy current effect on the coil. It would also beadvantageous to provide more than one of the return electrodes describedherein and provide a switch mechanism to switch between or utilize morethan one. This way a user could select the configuration that is mostcomfortable for them.

FIG. 8 shows a cross sectional view of a cochlear electrode arrayaccording to the present invention wherein the electrodes 174 stick outof the polymer isolating layer. FIG. 8A shows a cross sectional view ofa cochlear electrode array with recessed electrodes and silicone bumpsbetween the electrodes according to the present invention with elevatedelectrodes. The obtained electrode array contains micro sticks orfillings of polymer, especially such containing silicone in the spacesbetween the electrodes. The edges of the electrodes can be covered by agrid or mesh of polymer to increase the adhesion and stability. Polymercan contain polyimide, silicone, peek or parylene, or mixtures thereof.Gaps can be filled in by polymer such as PDMS or epoxy wherein a platedsoft low stress layer can be provided. It has been found that thesilicone adhesion to the polyimide is better when small holes areapplied to the polyimide surface prior to applying of silicone on thatsurface. FIG. 8B shows a top view of a cochlear electrode with multipleleads. The two leads or traces on the right end left side of theelectrode array 176A and 176B lead to electrodes preferably on differentlevels of electrodes (176A layer 1 and 176B layer 2). FIG. 8C shows adifferent variation of the multi lead of the trace wires, wherein theupper half and lower half are cut vertically and are in different layers(176A layer 1 and 176B layer 2). FIG. 8D shows again a differentvariation wherein the two halves are placed next to each other (176Alayer 1 and 176B layer 2). The traces require this way a smaller widththan traces without multi leads.

FIG. 11 show the preferred cochleostomy grommet. The grommet includes anupper half 10 and a lower half 12, joined by a hinge 14. Preferably, thecochleostomy grommet is made from a single piece of biocompatiblepolymer such as polyimide and sufficiently narrowed at the hinge 14 tomake it flexible. A snap type closure 16 is opposite the hinge. Inaddition, the grommet may include a grove 18, to facilitate tying asuture around the grommet.

FIGS. 9 and 10 show perspective views of the implanted portion of thepreferred cochlear prosthesis with a grommet, 100 which is in place.FIG. 12 shows the cochlear prosthesis attached by the grommet to thecochleostomy.

FIG. 13 and 14 show the implanted electrode array with a soft tip whichpreferably contains silicone. During insertion of the array into thecochlea, it is easy to cause surgical trauma. Silicone skirt 160 can beextended at the tip of the array to provide extra protection.

FIGS. 15-17 show a cross-section of multilayer structure. The traces(metal) are in more than one layer, preferably two or three layers. Thisvariation allows a reduced width of the conducting part of the electrodearray. The advantage is that the conducting density is increased. Theconducting part can be made thinner by slight increase of the depth. Theincrease of the depth has only minor influence on the flexibility of theconducting part. FIG. 15 shows that the wire traces are separated byinsulating polymer containing polyimide. FIG. 17 shows filled vialeading into a conducting pad. The current enters the electrode throughvia in the center and not at the edge. If the current is led to theelectrode at the edge and the electrode starts to dissolve at the edgeto the insulating material it leads to a faster break up of theconductivity. The advantage of the present multilayer is that theconductivity stays stable because even if the electrode starts todissolve at the edge to the insulating material the conductivity staysstable until the electrode dissolves completely towards the center. Viascan be used to connect traces on different metal layers or to connect anelectrode to a metal trace.

FIG. 16 depicts a cross-sectional view of a multilayer structure. Itshows as an example two layers of traces T and T2. Trace T has a directcontact with an electrode. Trace T2 also has a direct contact to theelectrode E.

FIG. 17 depicts a cross-sectional view of a multilayer structure withinter layer vias. Vias allow connections between multiple conductorlayer, here metal traces. FIG. 17 shows that via passes current throughthe center of the electrode and not from the edge of the electrode. Thishas a certain advantage because the edge of the electrode has thetendency to dissolve faster.

FIG. 18 shows a top view of a cochlear prosthesis molded into a helicalshape. The cochlear electrode array which preferably contains polyimidecan be molded in a silicone mold in a helix structure. The helixstructure is advantageous to be implanted into the cochlear.

The implanted portion of the preferred cochlear prosthesis is preferablycoated with a lubricious and/or hydrophilic coating, comprisinghydrophilic polymer containing poly(N-vinyl lactams,poly(vinylpyrrolidone), poly(ethylene oxide), poly(propylene oxide),polyacrylamides, cellulosics, methyl cellulose, polyanhydrides,polyacrylic acids, polyvinyl alcohols, polyvinyl ethers or mixturesthereof.

The above descriptions have been intended to illustrate the preferredand alternative embodiments of the invention. It will be appreciatedthat modifications and adaptations to such embodiments may be practicedwithout departing from the scope of the invention, such scope being mostproperly defined by reference to this specification as a whole and tothe following claims.

The invention claimed is:
 1. A cochlear electrode array suitable forcochlear stimulation comprising: a bottom polymer layer; a first layerof patterned high density metal traces deposited on said bottom polymerlayer; a polymer interlayer deposited on said bottom polymer layer andsaid first layer of patterned high density metal traces defining voidstherein for electrodes or connections; a second layer of patterned highdensity metal traces deposited on said polymer interlayer; a top polymerlayer deposited on said polymer interlayer and on said second layer ofpatterned high density metal traces defining voids therein forelectrodes; electrodes deposited in said voids; said electrodes having asurface coating; wherein said top polymer layer defines openings smallerthan said patterned high density metal trace; and said cochlearelectrode array is curved into a three-dimensional helical shape toapproximate the three-dimensional shape of the cochlea.
 2. The cochlearelectrode array of claim 1 wherein said top polymer layer hasoverlapping edges and mesh grids over said electrodes to concentratecurrent delivery.
 3. The cochlear electrode array of claim 1 whereinsaid surface coating is a gradient coating comprising platinum grey andiridium oxide.
 4. The cochlear electrode array of claim 1 wherein saidelectrodes are arranged linearly along said cochlear electrode array. 5.The cochlear electrode array of claim 1 wherein said electrode array hasat least one common electrode in the form of one of a mesh, star patternor hash pattern.
 6. The cochlear electrode array of claim 1 furthercomprising multiple common electrodes and a switch mechanism to switchbetween said multiple common electrodes.
 7. The cochlear electrode arrayof claim 6 further comprising vias to connect said patterned highdensity metal traces on different layers.
 8. The cochlear electrodearray of claim 1 further comprising a narrowed portion of said cochlearelectrode array.
 9. The cochlear electrode array of claim 1 furthercomprising more than one layer of wire traces separated by one or morelayers of an insulating polymer.
 10. The cochlear electrode array ofclaim 1 further comprising a coating comprising a hydrophilic polymerselected from the group containing poly(N-vinyl lactams,poly(vinylpyrrolidone), poly(ethylene oxide), poly(propylene oxide),polyacrylamides, cellulosics, methyl cellulose, polyanhydrides,polyacrylic acids, polyvinyl alcohols, polyvinyl ethers or mixturesthereof.
 11. The cochlear electrode array of claim 1 further comprisinga cochleostomy grommet which includes an upper half and a lower halfjoined by a hinge.
 12. The cochlear electrode array of claim 11, whereinsaid cochleostomy grommet comprises a single piece of biocompatiblepolymer.
 13. The cochlear electrode array of claim 1 wherein said tracescomprise platinum as a conductive layer.
 14. The cochlear electrodearray of claim 1 further comprising a skirt comprising a polymersurrounding the array and extending beyond its edges.
 15. The cochlearelectrode array of claim 1 wherein said patterned high density metaltraces further comprises adhesion layers on top and bottom sides of saidpatterned high density metal traces.
 16. The cochlear electrode array ofclaim 15 wherein said adhesion layers comprise titanium.
 17. Thecochlear electrode array of claim 1 wherein said a skirt of a morecompliant polymer surrounds said cochlear electrode array.